A controllable single-photon source is an optical source that emits, with high probability, one and only one photon in response to an external triggering event. Controllable single-photon sources are of interest for applications in quantum information processing, quantum cryptography, and quantum computation.
Controllable single-photon sources are typically based on the preparation of a quantum system, such as an atom or a molecule, in an excited state that can make a radiative transition to a lower energy state by emitting a single photon. For example, a two level system, or an atom or molecule that effectively acts as a two level system, can be prepared in an excited state by appropriate on-resonance optical pumping. Here on-resonance means the pump photon energy is equal to the two level transition energy, and is thus also equal to the emitted photon energy. Since a quantum system that is continuously driven by an on-resonance pump will emit a succession of photons at random times, a non-continuous pumping scheme is required. Pumping an atom or molecule with a short, intense pulse of on-resonance pump light is, at least conceptually, a simple method for providing a single-photon source.
However, this approach has significant practical drawbacks. The first drawback is that intense coherent optical pumping of a two level system leads to Rabi oscillations, where the probability of occupancy of the upper and lower states are sin2(Ωt) and cos2(Ωt) respectively, where Ω is the Rabi frequency and t is time. The Rabi frequency depends, in part, on the optical pump intensity. Thus, in order to prepare a quantum system in its upper level, ΩTp must equal π (or an odd multiple thereof), where Tp is the pulse duration. Furthermore, the pulse duration Tp must be less than the upper level dephasing time. A pump pulse satisfying this condition is referred to as a “π pulse”. Such pulses are not easy to provide in practice, since a particular relation between pulse intensity and duration must be satisfied. More precisely, the time-integral of a certain function of the optical electric field over the pulse duration must equal π or an odd multiple of π. Another drawback of this conceptually simple approach is that the pump radiation and single-photon radiation have the same wavelength, which complicates the task of separating the single-photon radiation from the residual pump radiation.
Thus experimental demonstrations of single-photon sources have followed other approaches. For example, a “turnstile” effect based on a Coulomb blockade for electrons and holes in a mesoscopic double-barrier p-n junction has provided a single-photon source (Nature, 397, 500–503, 1999). However, this experiment had to be performed at an exceedingly low temperature (i.e. 50 mK), and the sample geometry made collection of single-photon light difficult (i.e., the detection efficiency was about 1 part in 10−4).
Another experimental demonstration made use of rapid adiabatic following to prepare a molecule in an excited state (Brunel et al., Physical Review Letters, 83(14), 2722–2725, 1999). In rapid adiabatic following, continuous-wave pumping is employed, but the sample and pump are swept through the on-resonance condition, either by changing the pump photon energy or by altering the resonant energy of the quantum system within the sample (e.g., by applying a secondary electric field to Stark shift the relevant optical transition). Rapid adiabatic following provides less critical conditions on the pumping parameters than the use of π pulse pumping.
In this experiment, the active molecule was dibenzanthanthrene in an n-hexadecane matrix, an RF electric field was applied to the sample to Stark shift its transition energy relative to the pump photon energy, the sample temperature was 1.8 K, and the detection efficiency was about 3×10−3. Low temperatures were required to force the optical absorption line to be extremely narrow, in order to be able to Stark shift the transition energy by a significant fraction of the absorption linewidth with experimentally accessible secondary electric fields. The absorption linewidth is about 104 to 105 times larger at room temperature than at temperatures <4K. Thus, attempting to perform the experiment of Brunel at room temperature would require increasing the RF electric field by the same factor (i.e., 104 to 105), since the Stark shift is typically proportional to electric field. Such large electric fields are difficult or even impossible (if electric breakdown occurs) to provide in practice.
Disadvantages of this approach include low sample temperature and required narrow absorption making it difficult to implement this approach at temperatures greater than 10K. Moreover, the requirement of maintaining the sample in a cryostat contributes to the low detection efficiency. Adiabatic following has also been proposed, although not experimentally demonstrated, for a single-photon source including an atom that must be strongly coupled to a cavity (Applied Physics B, 69, 373–377, 1999).
A common feature of the above experimental approaches is that the optical excitation is on-resonance with a purely electronic transition from a ground state to an excited electronic state. As a consequence of this, the wavelength(s) of single-photon emission include the pump wavelength. Another common feature of the above approaches is that they are all coherent. More specifically, in these approaches, the state of the quantum system evolves in time according to the equations of density matrix quantum mechanics for all times between the beginning of pumping and the emission of a photon responsive to the pumping. If this coherent time evolution is interrupted by an external perturbation, such as a thermal perturbation, the desired processes leading to single photon emission tend to be disrupted. For example, even if an RF electric field sufficient to attempt the experiment of Brunel et al. at room temperature were provided, the performance of such an arrangement as a single photon source would be greatly inferior to its performance at cryogenic temperatures. This requirement of coherent time evolution is the basic reason why the above experimental results were only obtainable at ultra-low temperatures.
Therefore, there is an unmet need in the art for a room temperature single-photon source, and also for such a source having distinct pump and emission wavelengths.